Systems and methods for measuring three-dimensional motions of targets mounted to a moving subject, e.g., a human being, using stereo photogrametric techniques, are well known. In one such technique, video cameras connected to be controlled by a computer are employed to record two-dimensional motions of flat round or spherical targets. Such systems and methods may be utilized in biomechanics analyses in which the subjects are human beings or animals and may even be utilized to study multi-dimensional motions of robots, automobiles subjected to complex external forces, and the like. If two or more video cameras are employed to determine the two-dimensional motion of an observed target, ample computer software is available for calculating an overall three-dimensional motion and, therefore, successive locations of the target.
A basic underlying principle in the automation of two-dimensional target measurements of this type is that the targets be the brightest objects within the field-of-view of each of the cameras. This requirement allows for electronic discrimination of the targets themselves from the subject to which they are mounted as well as the background, using for example a simple threshholding technique on the output video signal from each camera. Typically, a plurality of targets are mounted to the moving subject and are illuminated by bright lights which are usually strobed to freeze the observed target motion. The targets are covered with a highly-reflective material coating and the cameras are pointed toward the targets. Such systems are termed "passive target systems", because the targets do not themselves emit any light but merely reflect it. In such studies, the subject is normally covered in clothing and the subject's skin and clothing each have a lower reflectance capacity than do the targets.
A known problem associated with the passive target technique, due to the required proximity of the subject to the camera and the light source, is that the subject itself may reflect enough light at a level which exceeds the target detection threshold and perhaps even the saturation point on the camera in the system. When this happens, the camera and the computer with which it communicates will receive information not accurately associated with specific target positions. This causes confusion and corrupts the data. One solution that has been tried to overcome this problem is to use the lens F/stop on the camera to decrease the amount of light received by the camera within its field-of-view. This solution, however, is not entirely satisfactory as the desired measurement accuracy requires procedures for the correction of lens nonlinearities, and the modification of the lens characteristics in any such way is best avoided.
In using two cameras to obtain three-dimensional motion data, it is also very important to be able to freely mount and position the cameras and their strobe lighting units on opposite sides of the subject while avoiding the problems that can arise if each of the cameras can image the other camera's strobe lights.
Among the various solutions that have been proposed is the teaching in U.S. Pat. No. 4,199,253, to Ross, namely the use of pulsed radiant lights and three cameras for three-dimensional measurements. The disclosed device utilizes a clock-pulse generator to trigger a light source and the three cameras have separately operated shutters so that light is reflected from spaced-apart zones to the viewed object and reflected light therefrom is recorded.
U.S. Pat. No. 4,951,073 to Slavitter, teaches the use of two conventional cameras, of which one may carry instant film and the other a typical negative film. Actuation of the two cameras is controlled by a synchronizing device so that the shutter of each device is open when the strobe or flash unit of one of the cameras or one of the more remote strobe or flash units controlled by one of the cameras is actuated. Both shutters are opened when the flash fires.
U.S. Pat. No. 4,882,498 to Cochran et al., teaches the use of a primary light source and a ring array of secondary light sources comprising light-emitting elements. Triggering of the light emitting elements is controlled by a computer through a strobe controller. Three-dimensional observations are performed by firing selected combinations of the light emitting elements.
U.S. Pat. No. 4,717,952 to Kohayakawa et al., discloses a medical television system employing a television camera and a selector, the camera itself including a filter for separating visible light into three wavelength regions and transmitting near-infrared rays. The camera also includes an image pickup element sensitive to both visible light and to near-infrared rays. The selector selectively supplies the visible light which is separated by the filter into the three selected wavelength components or the near-infrared rays, to a television camera.
U.S. Statutory Invention Registration No. H12, to Bennett et al., discloses a system employing two large field-of-view scintillation cameras mounted on a rotatable gantry, the cameras being movable diametrically toward or away from each other with respect to the subject. In addition, each of the cameras may be rotated about an axis perpendicular to the diameter of the gantry along which the cameras are movable. By using the two cameras at an angle to each other, improved sensitivity and depth resolution are obtained.
Although the above-discussed references and others teach various solutions, there remains a need for a kinematic data collection tool and a method employing readily available individual components in a system which is simple to operate and readily adaptable to a variety of data collection conditions while maintaining a high level of accuracy.